Atomistic View of the Energy Transfer in a Fluorophore-Functionalized Gold Nanocluster.

Understanding the dynamics of Förster resonance energy transfer (FRET) in fluorophore-functionalized nanomaterials is critical for developing and utilizing such materials in biomedical imaging and optical sensing applications. However, structural dynamics of noncovalently bound systems have a significant effect on the FRET properties affecting their applications in solutions. Here, we study the dynamics of the FRET in atomistic detail by disclosing the structural dynamics of the noncovalently bound azadioxotriangulenium dye (KU) and atomically precise gold nanocluster (Au25(p-MBA)18, p-MBA = para-mercaptobenzoic acid) with a combination of experimental and computational methods. Two distinct subpopulations involved in the energy transfer process between the KU dye and the Au25(p-MBA)18 nanoclusters were resolved by time-resolved fluorescence experiments. Molecular dynamics simulations revealed that KU is bound to the surface of Au25(p-MBA)18 by interacting with the p-MBA ligands as a monomer and as a π-π stacked dimer where the center-to-center distance of the monomers to Au25(p-MBA)18 is separated by ∼0.2 nm, thus explaining the experimental observations. The ratio of the observed energy transfer rates was in reasonably good agreement with the well-known 1/R6 distance dependence for FRET. This work discloses the structural dynamics of the noncovalently bound nanocluster-based system in water solution, providing new insight into the dynamics and energy transfer mechanism of the fluorophore-functionalized gold nanocluster at an atomistic level.

Nanoscale Probing of the Supramolecular Assembly in a Two-Component Gel by Near-Field Infrared Spectroscopy.

The design of soft biomaterials requires a deep understanding of molecular self-assembly. Here a nanoscale infrared (IR) spectroscopy study of a two-component supramolecular gel is introduced to assess the system's heterogeneity and supramolecular assembly. In contrast to far-field IR spectroscopy, near-field IR spectroscopy revealed differences in the secondary structures of the gelator molecules and non-covalent interactions at three distinct nano-locations of the gel network. A ß-sheet arrangement is dominant in single and parallel fibres with a small proportion of an α-helix present, while the molecular assembly derives from strong hydrogen bonding. However, at the crossing point of two fibres, only the ß-sheet motif is observed, with an intense π-π stacking contribution. Near-field nanospectroscopy can become a powerful tool for the nanoscale distinction of non-covalent interactions, while it is expected to advance the existing spectroscopic assessments of supramolecular gels.

Diversity at the nanoscale: laser-oxidation of single-layer graphene affects Fmoc-phenylalanine surface-mediated self-assembly.

We report the effects of a laser-oxidized single layer graphene (SLG) surface on the self-assembly of amphiphilic gelator N-fluorenylmethoxycarbonyl-L-phenylalanine (Fmoc-Phe) towards an gel-SLG interface. Laser oxidation modulates the levels of hydrophobicity/hydrophilicity on the SLG surface. Atomic force, scanning electron, helium ion and scattering scanning nearfield optical microscopies (AFM, SEM, HIM, s-SNOM) were employed to assess the effects of surface properties on the secondary and tertiary organization of the formed Fmoc-Phe fibres at the SLG-gel interface. S-SNOM shows sheet-like secondary structures on both hydrophobic/hydrophilic areas of SLG and helical or disordered structures mainly on the hydrophilic oxidized surface. The gel network heterogeneity on pristine graphene was observed at the scale of single fibres by s-SNOM, demonstrating its power as a unique tool to study supramolecular assemblies and interfaces at nanoscale. Our findings underline the sensitivity of assembled structures to surface properties, while our characterization approach is a step forward in assessing surface-gel interfaces for the development of bionic devices.

Influence of Fano resonance on SERS enhancement in Fano-plasmonic oligomers.

Plasmonic oligomers can provide profound Fano resonance in their scattering responses. The sub-radiant mode of Fano resonance can result in significant near-field enhancement due to its light trapping capability into the so-called hotspots. Appearance of these highly localized hotspots at the excitation and/or Stokes wavelengths of the analytes makes such oligomers promising SERS active substrates. In this work, we numerically and experimentally investigate optical properties of two disk-type gold oligomers, which have different strength and origin of Fano resonance. Raman analysis of rhodamine 6G and adenine with the presence of the fabricated oligomers clearly indicates that an increment in the strength of Fano resonance can improve the Raman enhancement of an oligomer significantly. Therefore, by suitable engineering of Fano lineshape, one can achieve efficient SERS active substrates with spatially localized hotspots.

Dynamics behind the long-lived coherences of I2 in solid Xe.

The absorption spectrum of I2 in solid Xe shows resolved zero-phonon lines and phonon side bands near the origin of the B←X transition (550-625 nm). The long-lived |B⟩⟨X| coherence in this energy range (T2 = 600 fs on average) emerges as vibrationally unrelaxed fluorescence in resonance Raman (RR) spectra. Upon excitation in the structureless continuum at 532 nm, the oscillatory RR progression exhibits electronic dephasing time of T2 = 150 fs. Two RR progressions with markedly different vibrational coherence on the X-state are observed. The main progression of sharp overtones (T2 > 21 ps) is assigned to molecules trapped in double-substitution sites. The minor progression, which shows dephasing times T2 = 6-0.6 ps for v = 1-8, is assigned to molecules in triple-substitution sites. The line progressions allow a detailed characterization of the solvated B- and X-state potentials. Time-resolved coherent anti-Stokes Raman scattering is used to probe selected vibrational coherences on the X-state. Assignments are obtained through molecular dynamics simulations, which reproduce the relative dephasing rates between the two sites, clarify the role of rotation-translation dynamics, and enable quantum dynamics simulations of the spectra by the potentials of mean force that accurately describe the molecule-surrounding interactions.

Electronic spectroscopy of I(2)-Xe complexes in solid Krypton.

In the present work, we have studied ion-pair states of matrix-isolated I(2) with vacuum-UV absorption and UV-vis-NIR emission, where the matrix environment is systematically changed by mixing Kr with Xe, from pure Kr to a more polarizable Xe host. Particular emphasis is put on low doping levels of Xe that yield a binary complex I(2)-Xe, as verified by coherent anti-Stokes Raman scattering (CARS) measurements. Associated with interaction of I(2) with Xe we can observe strong new absorption in vacuum-UV, redshifted 2400 cm(-1) from the X → D transition of I(2). Observed redshift can be explained by symmetry breaking of ion-pair states within the I(2)-Xe complex. Systematic Xe doping of Kr matrices shows that at low doping levels, positions of I(2) ion-pair emissions are not significantly affected by complexation with Xe, but simultaneous increase of emissions from doubly spin-excited states indicates non-radiative relaxation to valence states. At intermediate doping levels ion-pair emissions shift systematically to red due to change in the average polarizability of the environment. We have conducted spectrally resolved ultrafast pump-probe ion-pair emission studies with pure and Xe doped Kr matrices, in order to reveal the influence of Xe to I(2) dynamics in solid Kr. Strikingly, relaxed emission from the ion-pair states shows no indication of complex presence. It further indicates that the complex escapes detection due to a non-radiative relaxation.

Tailoring the interaction between a gold nanocluster and a fluorescent dye by cluster size: creating a toolbox of range-adjustable pH sensors.

We present a novel strategy for tailoring the fluorescent azadioxatriangulenium (KU) dye-based pH sensor to the target pH range by regulating the pK a value of the gold nanoclusters. Based on the correlation between the pK a and surface curvature of ligand-protected nanoparticles, the pK a value of the gold nanoclusters was controlled by size. In particular, three different-sized para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters, Au25(p-MBA)18, Au102(p-MBA)44, and Au210-230(p-MBA)70-80 were used as the regulator for the pH range of the KU response. The negatively charged gold nanoclusters enabled the positively charged KU to bind to the surface, forming a complex and quenching the fluorescence of the KU by the energy transfer process. The fluorescence was restored after adjusting the surface charge of the gold nanocluster by controlling the solution pH. In addition, the KU exhibited a significantly different pH response behaviour for each gold nanocluster. Au210-230(p-MBA)70-80 showed a higher pH response range than Au102(p-MBA)44, which was intuitive. However, Au25(p-MBA)18 showed an unexpectedly high pH response behaviour. pK a titration measurement, molecular dynamics simulations, and essential dynamics analysis showed that small nanoclusters do not follow the scaling between the curvature and the pK a value. Instead, the behaviour is governed by the distribution and interaction of p-MBA ligands on the nanocluster surface. This work presents an effective design strategy for fabricating a range adjustable pH sensor by understanding the protonation behaviour of the ultrasmall gold nanoclusters in an atomic range.

Electronic and vibrational signatures of the Au102(p-MBA)44 cluster.

Optical absorption of a gold nanocluster of 102 Au atoms protected by 44 para-mercaptobenzoic acid (p-MBA) ligands is measured in the range of 0.05-6.2 eV (mid-IR to UV) by a combination of several techniques for purified samples in solid and solution phases. The results are compared to calculations for a model cluster Au(102)(SMe)(44) based on the time-dependent density functional theory in the linear-response regime and using the known structure of Au(102)(p-MBA)(44). The measured and calculated molar absorption coefficients in the NIR-vis region are comparable, within a factor of 2, in the absolute scale. Several characteristic features are observed in the absorption in the range of 1.5-3.5 eV. The onset of the electronic transitions in the mid-IR region is experimentally observed at 0.45 ± 0.05 eV which compares well with the lowest calculated transition at 0.55 eV. Vibrations in the ligand layer give rise to fingerprint IR features below the onset of low-energy metal-to-metal electronic transitions. Partial exchange of the p-MBA ligand to glutathione does not affect the onset of the electronic transitions, which indicates that the metal core of the cluster is not affected by the ligand exchange. The full spectroscopic characterization of the Au(102)(p-MBA)(44) reported here for the first time gives benchmarks for further studies of manipulation and functionalization of this nanocluster to various applications.

Rotational coherence imaging and control for CN molecules through time-frequency resolved coherent anti-Stokes Raman scattering.

Numerical wave packet simulations are performed for studying coherent anti-Stokes Raman scattering (CARS) for CN radicals. Electronic coherence is created by femtosecond laser pulses between the X(2)Σ and B(2)Σ states. Due to the large energy separation of vibrational states, the wave packets are superpositions of rotational states only. This allows for a specially detailed inspection of the second- and third-order coherences by a two-dimensional imaging approach. We present the time-frequency domain images to illustrate the intra- and intermolecular interferences, and discuss the procedure to rationally control and experimentally detect the interferograms in solid Xe environment.

Covalent and non-covalent coupling of a Au102 nanocluster with a fluorophore: energy transfer, quenching and intracellular pH sensing.

Interactions between an atomically precise gold nanocluster Au102(p-MBA)44 (p-MBA = para mercaptobenzoic acid) and a fluorescent organic dye molecule (KU, azadioxatriangulenium) are studied. In solution, the constituents form spontaneously a weakly bound complex leading to quenching of fluorescence of the KU dye via energy transfer. The KU can be separated from the complex by lowering pH, leading to recovery of fluorescence, which forms a basis for an optical reversible pH sensor. However, the sensor is not a stable entity, which could be delivered inside cells. For this purpose, a covalently bound hybrid is synthesized by linking the KU dye to the ligand layer of the cluster via an ester bond. Covalent linking facilitates entry of the cluster-dye hybrids into cells via endocytosis. Inside cells, the hybrids accumulate in endosomes where Au102 releases its cargo via hydrolysis of the ester bond. Changes of the local pH inside endosomes regulate reversible fluorescence due to variations in the interactions between the Au102 cluster and the dye. This work presents a concept for delivering reporter molecules into cells by using atomically precise gold nanoclusters as carriers and paves the way for future developments of cluster-reporter sensors for in vivo measurements of e.g. absolute pH values or ion concentrations.

Electrical Control of Interband Resonant Nonlinear Optics in Monolayer MoS2.

Monolayer transition-metal dichalcogenides show strong optical nonlinearity with great potential for various emerging applications. Here we demonstrate the gate-tunable interband resonant four-wave mixing and sum-frequency generation in monolayer MoS2. Up to 80% modulation depth in four-wave mixing is achieved when the generated signal is resonant with the A exciton at room temperature, corresponding to an effective third-order optical nonlinearity |χ(3)eff| tuning from (∼12.0 to 5.45) × 10-18 m2/V2. The tunability of the effective second-order optical nonlinearity |χ(2)eff| at 440 nm C-exciton resonance wavelength is also demonstrated from (∼11.6 to 7.40) × 10-9 m/V with sum-frequency generation. Such a large tunability in optical nonlinearities arises from the strong excitonic charging effect in monolayer transition-metal dichalcogenides, which allows for the electrical control of the interband excitonic transitions and thus nonlinear optical responses for future on-chip nonlinear optoelectronics.

Chemically Selective Imaging of Individual Bonds through Scanning Electron Energy-Loss Spectroscopy: Disulfide Bridges Linking Gold Nanoclusters.

As proof-of-principle of chemically selective, spatially resolved imaging of individual bonds, we carry out electron energy-loss spectroscopy in a scanning transmission electron microscopy instrument on atomically precise, thiolate-coated gold nanoclusters linked with 5,5'-bis(mercaptomethyl)-2,2'-bipyridine dithiol ligands. The images allow the identification of bridging disulfide bonds (R-S-S-R) between clusters, and X-ray photoelectron spectra support the finding.

From monomer to bulk: appearance of the structural motif of solid iodine in small clusters.

Formation of iodine clusters in a solid krypton matrix was studied using resonance Raman spectroscopy with a 1 cm(-1) resolution. The clusters were produced by annealing of the solid and recognized by appearance of additional spectral transitions. Two distinct regions, red-shifted from the fundamental vibrational wavenumber of the isolated I(2) at 211 cm(-1), were observed in the signal. The intermediate region spans the range 196-208 cm(-1), and the ultimate region consists of two peaks at 181 and 190 cm(-1) nearly identical to crystalline I(2). The experimental results were compared to DFT-D level electronic structure calculations of planar (I(2))(n) clusters (n = 1-7). The dimer, trimer, and tetramer structures, where the I(2) molecule is complexed from one end, were found to exhibit vibrational shifts corresponding to the intermediate size clusters. The larger, bulklike shift appears when the iodine molecule is coordinated from two opposite directions as in the case of a pentamer and higher clusters. Starting from the pentamer, the structural motif of crystalline iodine is clearly recognized in the clusters.

Iodine-benzene complex as a candidate for a real-time control of a bimolecular reaction. Spectroscopic studies of the properties of the 1:1 complex isolated in solid krypton.

The properties of the 1:1 iodine-benzene complex isolated in a solid Kr matrix at low temperatures have been studied using UV-vis absorption, FTIR, resonance Raman, and femtosecond coherent anti-Stokes Raman spectroscopy (fs-CARS). The use of all these techniques on similar samples provides a wide view on the spectroscopic properties of the complex and allows comparison and combination of the results from different methods. The results for the complex cover its structure, the changes in the iodine molecule's vibrational frequencies and electronic absorption spectrum upon complexation, and the dynamics of the complexed I(2) molecule on both ground and excited electronic states. In addition, polarization beats between uncomplexed benzene and iodine molecules are detected in the fs-CARS spectra, showing an amplification of an electronically nonresonant CARS signal by the resonant iodine signal. The possibility of controlling the charge-transfer reaction of the I(2)-Bz complex using the excitation of a well-defined ground-state vibrational wavepacket, according to the Tannor-Rice-Kosloff scheme, is discussed on the basis of the experimental findings.

Vibrational characterization of the 1:1 iodine-benzene complex isolated in solid krypton.

The structure and properties of a 1:1 iodine-benzene complex isolated in an inert krypton matrix at low temperature have been studied with infrared and resonance Raman spectroscopy and with MP2 calculations. The structure of the ground-state complex is found to be unsymmetric, and the I-I vibrational frequency is found to be red-shifted by 3.94 cm(-1) upon complexation. The experimental data agree well with computational results, leading to the conclusion that the I2-Bz complex structure is not axial but of above-bond type, identically with other halogen-benzene complexes.

Dithiol-Induced Oligomerization of Thiol-Protected Gold Nanoclusters.

Controlled synthesis of nanostructure oligomers requires detailed understanding of their wet chemistry and the forces driving the polymerization process. In this paper, we report the main factors affecting the reaction yields of a dithiol-induced synthesis of covalently bound nanocluster dimers and oligomers and present a detailed analysis of possible reaction mechanisms. We synthesize the nanocluster oligomers using monodisperse para-mercaptobenzoic acid (p-MBA)-protected gold nanoclusters with a nominal composition of Au∼250(p-MBA) n to minimize ensemble effects on size, shape, and surface structure. Ligand exchange was performed on the nanoclusters with five different dithiol linkers: 5,5'-bis(mercaptomethyl)-2,2'-bipyridine, 4,4″-thiobisbenzenethiol, benzene-1,4-dithiol, 1,4-benzenedimethanethiol, and dimercaptostilbene. Oligomer yields depend strongly on the used dithiol and on the dithiol-to-nanocluster ratio. Detailed analysis of the reaction yields in combination with simulations suggests that the system reaches a dynamic equilibrium, where ligand exchange happens continuously forming and breaking nanocluster oligomers that are bound together by short chains of disulfide-bridged dithiols. Despite the dynamic nature of the system, dithiol-induced polymerization of nanoclusters is a general and straightforward approach to produce dimers and larger oligomers of thiol-protected nanoclusters. Our work provides physical insight into, offers tools for, and reveals challenges in the controlled synthesis of covalently bound nanoparticle assemblies.

Covalently linked multimers of gold nanoclusters Au102(p-MBA)44 and Au∼250(p-MBA)n.

We present the synthesis, separation, and characterization of covalently-bound multimers of para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters. The multimers were synthesized by performing a ligand-exchange reaction of a pre-characterized Au102(p-MBA)44 nanocluster with biphenyl-4,4'-dithiol (BPDT). The reaction products were separated using gel electrophoresis yielding several distinct bands. The bands were analyzed by transmission electron microscopy (TEM) revealing monomer, dimer, and trimer fractions of the nanocluster. TEM analysis of dimers in combination with molecular dynamics simulations suggest that the nanoclusters are covalently bound via a disulfide bridge between BPDT molecules. The linking chemistry is not specific to Au102(p-MBA)44. The same approach yields multimers also for a larger monodisperse p-MBA-protected cluster of approximately 250 gold atoms, Au∼250(p-MBA)n. While the Au102(p-MBA)44 is not plasmonic, the Au∼250(p-MBA)n nanocluster supports localized surface plasmon resonance (LSPR) at 530 nm. Multimers of the Au∼250(p-MBA)n exhibit additional transitions in their UV-vis spectrum at 630 nm and 810 nm, indicating the presence of hybridized LSPR modes. Well-defined structures and relatively small sizes make these systems excellent candidates for connecting ab initio theoretical studies and experimental quantum plasmonics. Moreover, our work opens new possibilities in the controlled synthesis of advanced monodisperse nanocluster superstructures.

Long-Lived Electronic Coherence of Iodine in the Condensed Phase: Sharp Zero-Phonon Lines in the B↔X Absorption and Emission of I2 in Solid Xe.

Our study of B←X absorption of molecular iodine (I2) isolated in a low-temperature crystalline xenon has revealed an exceptionally long-lived electronic coherence in condensed phase conditions. The visible absorption spectrum shows prominent vibronic structure in the form of zero-phonon lines (ZPLs) and phonon side bands (PSBs). The resolved spectrum implies weak interaction of the chromophore to the lattice degrees of freedom. The coherence extends past the vibrational period of the excited state molecule, unlike that observed in any condensed phase environment for I2 so far. The ZP transitions from the relaxing B-state populations were resolved in the hot luminescence when the 532 nm laser was used for excitation.